1. Field of the Invention
The present invention relates to a node device having a label switched path loop detection function and a label switched path loop detection method.
2. Description of the Background Art
In a network layer protocol in which IP (Internet Protocol) nodes are inter-connected on an NBMA (Non-Broadcast Multiple Access) network, there is a scheme for realizing high speed packet transfer called MPLS (Multi-Protocol Label Switching).
In the MPLS, specific “labels” are allocated to specific packet flows between nodes while a correspondence between an input side label and an output side label is stored at each node, and the label switching is carried out at each node by referring to this information, so that the IP processing can be omitted and a high speed packet transfer can be realized. For example, in the case where the link layer is ATM, VPI(Virtual Path Identifier)/VCI (Virtual Channel Identifier) will be used as a label. A route through which the packet flow is label switched will be referred to as a label switched path.
In the case of constructing the label switched path, the label allocation protocol is used. The label allocation protocol exchanges label allocation messages between nodes on a packet flow route in order to notify a correspondence between a flow identifier for identifying the packet flow in network global fashion and a label for identifying the packet flow in link local fashion.
Usually, every time an IP packet passes through a node, a value of an TTL (Time To Live) field (or a hop limit field or their equivalents) within its packet header is decremented by one, and the IP packet is not forwarded any further when the TTL or the like becomes 0. In this way it is possible to prevent a packet to stay in the network indefinitely even when there is a loop in the route.
However, for a packet to be transferred by the label switched path, the IP processing will not be carried out at an intermediate node so that it is impossible to decrement the TTL or the like, and consequently the packet will not leave the label switched path forever if there is a loop in the label switched path. For this reason, it is necessary to provide a label switched path loop detection mechanism or a label switched path loop prevention mechanism in the label allocation protocol.
To this end, in the conventional label allocation protocol, the following two methods are available: (1) a method in which an information on a hop count from an ingress router which originated a label allocation message with respect to the packet flow is included in the label allocation message, and it is judged that a loop is present when the hop count exceeds a threshold (see Doolan et al., “Tag Distribution Protocol”, Internet Draft, draft-doolan-tdp-spec-01.txt, May 1997); and (2) a method in which each node that forwards the label allocation message sequentially adds its own address to the label allocation message, and it is judged that a loop is present when the own address is already written in the received label allocation message (see Rosen et al., “A Proposed Architecture for MPLS”, Internet Draft, draft-rosen-mpls-arch-00.txt, July 1997).
However, the above described method (1) has been associated with a problem that it takes a considerable amount of time until the loop detection because the presence of loop cannot be ascertained until the hop count exceeds the threshold.
Note that in this method if a loop is actually formed or not cannot be ascertained, and even if a loop is actually formed, it is impossible to ascertain a node at which a loop is formed.
On the other hand, the above described method (2) has been associated with a problem that the size of the label allocation message becomes quite large.